hello we are leaving perth.australia on xmas day on our uk passports ,even though we are OZ citizens we have never got our OZ passports ,returning home UK for good and dont intend on returning to OZ, would there be any trouble at the airport with us using our UK PP.........I am starting to worry that they wont let us leave the country

I left Australia for good, never to return, in 1996. No problems with passports, when leaving.

But I had a hell of a job, when I did come back

No, you should be fine. You aren't coming in, so you don't need to prove your right to enter (for which you'd need a passport as your visas would be cancelled). If you aren't coming back, don't worry about it.
If you ever do come back in the future, even just for a holiday, you wil need passports though, no more visas.